Patient Support

Most surgeries in small rodents are analogous to those performed in dogs and cats. The small size of these patients makes surgery more challenging for several reasons. As surgical patients, rodents are especially prone to complications such as hypovolemia from blood loss, hypothermia, and renal and respiratory compromise. Because they also serve as a food source for predator species, fear, pain, and stress have profound effects on their survival during and after surgery. It is believed that the release of large amounts of catecholamines, which can have effects on anesthesia as well, is responsible for such stress-related death. Efforts should be made to minimize pain, fear, and stress during the hospitalization period. The provision of preemptive analgesic and antianxiety medications is a vital component of a successful outcome.

A preoperative database should include a complete blood count (CBC), plasma biochemical analysis, urinalysis, and whole-body radiographs, especially to evaluate the lungs, because they are especially prone to asymptomatic respiratory disease as well as to renal disease. Rats and mice frequently have subclinical mycoplasmal pneumonia. Although renal disease is common in older rodents, it can be difficult to obtain urine in a small rodent. Because they urinate frequently, however, one can put the rodent in a clean, dry, plastic or glass container, and a few drops of urine can then be collected from the container within a few minutes. This is all that is required to evaluate the urine by dipstick and determine the specific gravity. Proteinuria and dilute urine suggest underlying renal disease. At the very least, a preoperative database should include a packed cell volume, total protein, urine specific gravity, and a blood urea nitrogen (BUN) estimate by an azotemia test strip. These tests require only a few drops of blood and one drop of urine.

Rodents are physiologically unable to vomit, so a prolonged fast is not necessary.³⁵ They also have relatively low hepatic glycogen stores and are prone to developing hypoglycemia during a prolonged fast.^16,35 A short fast of 1 hour will allow the animal to clear food material from its mouth for oral procedures or to minimize the risk of an endotracheal tube carrying food into the trachea if intubation is anticipated. Administration of fluids containing dextrose (SC, IV, or IO) will help combat loss of energy stores. A prolonged fast has not been shown to decrease the volume of abdominal contents in these hindgut cellulose-fermenting species.

Rodents have a large body surface area/volume ratio predisposing to the development of hypothermia during long anesthetic and surgical procedures. A temperature monitoring probe is ideal for continuous evaluation of core body temperature. The body temperature of a rat can drop 18°F (10°C) after 20 minutes of anesthesia.¹⁷ Hypothermia decreases the metabolism and excretion of many anesthetic drugs. A short operative time—accomplished by having all the necessary equipment ready and accessible—will help to minimize the development of hypothermia. Alcohol should be avoided for skin preparation because the evaporative cooling will potentiate the development of hypothermia. Warm, sterile saline is as effective as alcohol in patient preparation. Alcohol is also flammable and can ignite if electrosurgery or laser is used. A circulating warm-water blanket under the patient should be set at 104°F (40°C), because these patients have a higher resting body temperature than dogs and cats. Forced warm air blankets (Bair Hugger, Arizant, Eden Prairie, MN) are also useful in combating hypothermia. The Thermally Controlled Surgery Pad (RICA Surgical Products, Inc., Schiller Park, IL) is a safe plastic pad placed under the patient that does not overheat and turns on automatically when a patient is placed on it and turns off automatically when not in use. Also, the Hot Dog Warming System (Augustine Biomedical & Design, Eden Prairie, MN) is a safe and effective conductive-fabric warming unit. Other sources of supplemental heat, including hot water bottles or examination gloves filled with warm water and radiant heat lamps, can cause thermal burns and must be used with caution. Placing a towel between the heat source and the patient does not ensure safety. Closely monitor all heat supplement devices to avoid accidentally burning the patient. The patient should be draped as quickly as possible because the drape will help hold in heat.

Seemingly small amounts of hemorrhage can be disastrous in such small patients. Although it may not be practical, ideally it is best to have blood from a conspecific available for transfusion when significant blood loss is anticipated. Strict attention to hemostasis is vital. The average cotton-tipped applicator holds approximately 0.1 mL of blood when completely soaked. Loss of 10% to 15% of the total blood volume (approximately 1% of body weight) is usually safe. Loss of more than five cotton-tipped applicators full of blood (0.5 mL, or 10 drops) in a 50-g mouse is equivalent to more than 20% of the blood volume and is potentially dangerous.¹⁶ Approximately 1.2, 1.4, and 4.0 mL of blood is equivalent to more than 20% of the blood volume (a potentially dangerous amount to lose) in the average-sized gerbil, hamster, and rat, respectively.¹⁶ Intravascular fluid volume may also be supported with the aid of crystalloid fluids such as lactated Ringer’s solution with or without dextrose. Intraosseous cannulas such as spinal needles are commonly inserted in a normograde fashion into the proximal femur or proximal tibia of pet rodents in a manner analogous to normograde insertion of an intramedullary pin for fracture management. This provides access to the vascular system for fluid support and emergency drug therapy if needed. Fluids may also be administered subcutaneously or intraperitoneally with slower absorption. These routes are not acceptable for treating animals that are seriously ill, severely dehydrated, or in shock. The standard recommendation for IV fluid support during anesthesia and surgery is 10 mL/kg per hour; however, a single dose of 10 mL/kg SC of 4% dextrose has been recommended for short procedures in rodents.³⁵

The thoracic cavity of many small rodents is small relative to their body size. Placing the patient in dorsal recumbency can compromise respiration because of the pressure of the abdominal viscera on the diaphragm. Tilting the patient so that the viscera are displaced caudally may be beneficial for the respiratory system of the anesthetized patient.³⁵ Securing the patient to a restraint board such as the Thermally Controlled Surgery Pad (Fig. 28-1) rather than the surgical table allows the patient to be tipped in any direction and to be easily repositioned intraoperatively.

Fig. 28-1 The Thermally Controlled Surgery Pad (RICA Surgical Products, Inc., Schiller Park, IL) provides safe, supplemental heat and serves as a patient restraint and positioning board. The pad, with the patient secured to it, can be tilted and moved during the surgery as needed.

Instrumentation

In general, ophthalmic instruments are not suited to surgery in small mammals (Fig. 28-2). The short length of the instruments makes them difficult to control and manipulate the tissues within the body cavity. Microsurgical instruments are constructed so that only the tips are miniaturized, which is different from ophthalmic instruments, where the whole instrument is miniaturized.⁴ The handles should be of normal length (5.5 to 7 in.) to help provide stability to the tips, thus diminishing the effects of tremors. The tips of the instruments should extend beyond the fingers far enough to use within the patient’s body cavity, while ophthalmic instruments may be used for surface work. The handles should be round to facilitate the required rolling action between the thumb and first two fingers. Round handles are most important for the needle holders, where a curved needle must be rolled through the tissue. Many prefer needle holders without a clasp or box lock because the motion that occurs when the lock is set and released may cause the needle to tear tissues. Hold a microsurgical instrument as if you were holding a pen. An across-the-palm grip is inappropriate and limits the function of these precision instruments (see Fig. 28-2).

Fig. 28-2 A, Ophthalmic instruments (right) are short, with flat finger grips, compared with microsurgical instruments, which are of a standard length and have rounded grips (left)B, They should be held like a pen. Microsurgical instruments are long enough to extend beyond the fingers into a body cavity and balance in the hand, requiring very little muscle action and thus reducing fatigue and tremor.

A microsurgical pack should consist minimally of a microsurgical needle holder (Micro Surgery Needle Holder, straight, T/C without lock, 8 in.; Micro Surgery Needle Holder, curved, without lock, 7.125 in., pencil-style grip), microsurgical scissors (Tew-Barraquer Scissors, curved, 7 in.), and microsurgical thumb forceps (Micro Surgical Forceps, curved, 7 in., pencil-style grip) (all from RICA Surgical Products, Inc., Schiller Park, IL). Many microsurgeons use jeweler’s forceps as microsurgical forceps (Long Jeweler’s Forceps No. 3, RICA Surgical Products, Inc.). These are significantly less expensive and serve their purpose adequately, but they are not ideal. They have sharp tips, are short, and do not have round handles. I prefer curved ring-tipped microsurgical forceps (Ring Tip Forceps, Cat. No. 843025, RICA Surgical Products, Inc.) and Counterweight Round Handle Ring Tip Tissue Forceps (Cat. No. 2200-260, Sontec Instruments, Centennial, CO) because they are of a standard length, counterbalanced, and delicate, but the tips have rings that distribute the pressure over a wider surface, making them less traumatic for tissue handling while one can still hold the fine strand of suture for knot tying (Fig. 28-3). Eyelid retractors do not work well in small rodents because the tension cannot be adjusted to fit the individual patient. Heiss and Alm Self Retaining Retractors (RICA Surgical Products, Inc.), analogous to Bennett and Doolan retractors (Sontec Instruments), respectively, work well as abdominal and tissue retractors in small rodents. The Lone Star Retractor (Veterinary Specialty Products, Inc., Mission, KS) consists of Silastic bands with tissue hooks on the ends and a plastic frame (Fig. 28-4). All components are fully autoclavable and quite durable. The stay hooks are placed in the tissue and the Silastic bands inserted into the notches of the frame to maintain tissue retraction. Use the least amount of tension necessary to maintain exposure. Hemostatic clips (Hemoclips and Samuels Hemoclip Applier, 6.25 in., RICA Surgical Products, Inc.) are also very useful and available in five sizes, with small and medium being most applicable for pet rodent surgery. A microsurgical pack can be put together for $1,500 to $2,000. After experience has been gained, other microsurgical instruments such as micro mosquito hemostats may be added to the pack.

Fig. 28-3 Jeweler’s forceps (bottom) have a very small point. Ring-tip forceps (top) have a fine tip that is broader and can be used to hold tissue as well as suture for knot tying.

Fig. 28-4 The Lone Star Retractor (Veterinary Specialty Products, Inc., Mission, KS) is autoclavable. The tissue hooks on Silastic bands are used to gently retract tissues during surgery. The hooks are placed into the tissue and the bands, under gentle tension, are wedged into the notches on the plastic frame.

Small gauze pads (2 × 2 in.) can be cut from the standard 4 × 4-in. sponges. Sterile cotton-tipped applicators should also be available. These are useful for absorption of fluids as well as gentle tissue dissection and manipulation. Absorbable gelatin sponge (Gelfoam, Pfizer, New York, NY; Vetspon, Novartis Animal Health US, Inc., Greensboro, NC) and other absorbable hemostatic agents are valuable for controlling hemorrhage. Surgicel (Ethicon, Inc., Sommerville, NJ) is oxidized regenerated cellulose resembling cloth. It is a hemostatic aid that adheres nicely to moist tissues but is not capable of absorbing much fluid. Topical thrombin (Thrombin-JMI, Gentrac, Inc. Middleton, WI) is available in various-sized vials. Thrombin is a liquid that stimulates fibrin clot formation using the patient’s fibrinogen. Either gelatin sponge or oxidized regenerated cellulose can be soaked in topical thrombin before being placed on the source of hemorrhage to aid in hemostasis.

Ophthalmic bulb syringes work well for tissue irrigation in small rodents, but a syringe with a needle attached is also applicable. Fine Barron suction tips have a release hole providing two degrees of suction. For rodent patients, the tip is used mainly with the hole open, providing suction that will not traumatize tissues. If there is a larger volume of fluid, occlude the hole with the tip of your index finger to increase the force of the suction.

Magnification

Some form of magnification is recommended for surgery in rodent patients. The dexterity and manipulation required of fingers and hands are far greater than can be achieved with unaided vision. Small amounts of hemorrhage appear more significant under magnification and individual vessels are much more easily identified for coagulation, minimizing the degree of hemorrhage associated with a procedure. Binocular loupes are available in various styles, with the hobby loupe being the least expensive and simplest. Hobby loupes have several disadvantages, including a set focal distance, no focal light source, and the inability to look around the lenses when magnification is not required.

A modification of the hobby loupe is marketed with interchangeable lenses and an LED focal light source (MDS, Inc., Brandon, FL). These allow the surgeon to change the degree of magnification as appropriate for the size of the patient and to illuminate body cavities. This type of loupe still has a set focal length and the surgeon is committed to looking through the lenses. The higher the power, the shorter the focal distance, and the closer the surgeon must be to the working area for objects to be in focus. These factors all contribute to dizziness in the wearer. These loupes also require the surgeon to bend his or her neck to be able to see through the lenses. This results in fatigue and neck pain.

SurgiTel (General Scientific Corp., Ann Arbor, MI) eliminates most of the problems associated with the hobby loupes (Fig. 28-5). It is ergonomically engineered to be used with a proper neck posture, the lenses do not cover the entire visual field so the surgeon is not committed to looking through the lenses, it has a focal LED light source, and the lenses also have a focal range, rather than a set focal distance, which generally allows the entire patient to be in focus during surgery.

Fig. 28-5 A, Surgitel (General Scientific Corp., Ann Arbor, MI) loupes have a focal range rather than a set focal distance and are available in various styles (glasses or mounted on a head band) with options for a focal light source. B, They are ergonomically designed for the surgeon to maintain an appropriate posture while providing magnification. The surgeon diverts the view down to use the magnification and over the lenses when magnification is not needed. The head and neck remain upright, reducing neck fatigue.

Focal Light

Overhead lights are not adequate for surgery in small rodents. A focal light provides illumination of a smaller area, allowing better visualization of the tissues. In working within a body cavity, a focal light is essential to illuminate the relevant structures. Various types of lights with or without magnifying loupes are available through various companies. LED lights are commonly used because they provide high-quality cool light with high intensity and durability. An inexpensive headband-mounted LED light, the LED Headlight, is available (MDS, Inc., Brandon, FL). SurgiTel loupes are available with different light options, including fiberoptic lights that adapt to endoscope light sources and halogen or LED lights that use a battery or wall outlet. These mount over the lenses and are adjusted to illuminate the surgeon’s field of view.

General Scientific Corp. also offers SurgiCam, a digital video camera with 3× magnification that mounts over the lenses. This system allows the procedure to be viewed on a monitor. It makes still photos or video recordings that can be edited—a useful client education tool.

Hemostatic Aids

Electrocautery refers to the use of electricity to heat metal until it is red hot to heat-coagulate tissues. Electrosurgery uses high-frequency alternating current to generate energy. There are two electrodes (an active electrode and an indifferent electrode) with concentration of current density at the tip of the smaller (active) electrode. Burns can result if the ground plate contacts only a small area. The Surgitron (Ellman International, Inc., Hewlett, NY) uses radiofrequency current that is received, with the indifferent electrode acting as an antenna. The area of contact between the patient and the indifferent electrode is irrelevant, which is especially advantageous in working with small mammal patients. The bipolar forceps is most useful for hemostasis within body cavities because the current passes between the tips of the electrodes, coagulating only the tissue held in the tips of the forceps. Ellman International offers a switch that allows the surgeon to change from monopolar (used for cutting the skin) to bipolar (used for coagulating vessels within a body cavity) and back without having to unplug electrodes.

The Surgitron Dual Frequency 120 (Ellman International, Inc., Hewlett, NY) radiowave energy is emitted at 4.0 MHz, providing control of absorption depth in tissue and resulting in minimal cellular damage and heat conduction (Fig. 28-6). This unit features a unique isolated circuitry system that maintains a constant frequency of 4.0 MHz. With this constant frequency, there is only 15 μm of lateral heat damage compared with 90 μm from the older Surgitron units and more than 100 μm with a carbon dioxide laser. Standard electrosurgical units generally produce more than 750 μm of lateral heat damage.

Fig. 28-6 The Surgitron Dual Frequency 120 radiosurgical unit (Ellman International, Inc., Hewlett, NY) is small and has both monopolar and bipolar capabilities. It operates with radiofrequency electromagnetic waves, with the ground plate acting as an antenna.

The finger switch has a button for cutting, another for coagulating, and a third for a blend of cutting and coagulating. A technician is not required to switch from bipolar to monopolar, as is required with the other electrosurgical units.

The carbon dioxide (CO₂) laser (Luxar and Aesculight, LuxarCare, Woodinville, WA) is popular in surgery involving small exotic animals. It produces a beam of light energy at a wavelength of 10,600 nm. This wavelength of energy is highly absorbed by water molecules, making it ideal for cutting with a focal beam or vaporizing tissue with a diffused beam. The CO₂ laser seals vessels less than 0.6 mm in diameter, so many of the incisions are bloodless, resulting in less postoperative swelling because the lymphatic vessels are also sealed. Incisions made with the CO₂ laser are reportedly less painful than those made with a blade or electrosurgical or radiosurgical units because the laser beam seals nerve endings as well. When used appropriately, the amount of lateral heat damage is said to be less with a laser than with some other modalities;⁴ however, when used incorrectly, lasers can cause significant lateral thermal damage.

Lasers can cause injury to the operator or assistant as well as to the other tissues of the patient. Proper safety training for all personnel, therefore, is important in using surgical lasers.

In the past several years, Harmonic Systems (Ethicon Endosurgery, Inc. Cincinnati, OH) has gained popularity for providing hemostasis during minimally invasive and open procedures. This is an ultrasonically activated tissue cutting device whereby electrical energy is converted to mechanical energy. Ultrasonic vibrations at the tip generate heat that results in tissue coagulation. After the tissue is coagulated and hemostasis is achieved, the device is reactivated to cut the tissue within the tips. It is approved by the U.S. Food and Drug Administration (FDA) to seal vessels up to 5 mm in diameter with less lateral heat than electrosurgery or laser. No electrical current passes through the patient. Electricity does not pass through the tissues, so muscles do not spasm when cut. The tissue is grasped between the Teflon anvil (cool) and the cutting blade (hot); it is then sealed at the low setting and cut at the high setting. The hand pieces are intended for single use but can be gas-sterilized and reused 10 to 15 times, making them affordable for veterinary practice.

LigaSure (Covidien, Mansfield, MA) is a tissue sealing system that has gained popularity in veterinary surgery. The newest model combines the tissue sealing technology with monopolar and bipolar electrosurgery (Force Triad). While the specific technology is proprietary, the LigaSure is a type of bipolar cautery. The generator senses the electrical impedance of the tissues within the tips and delivers the amount of current needed to melt the elastin and collagen; it then allows the elastin and collagen to re-form, creating a permanent seal in a single application. It is FDA-approved to seal vessels up to 7 mm in diameter and generates less than 1 mm of lateral heat. A translucent “seal zone” is created and the tissue is transected through this zone.

Patient Preparation

Standard aseptic technique is essential with pet rodent patients. Rodents are susceptible to infections and their cages often require them to be in close proximity to their urine and feces. Be careful to avoid damaging the skin during preparation for surgery. The area clipped should be minimized to help control heat loss. A No. 50 clipper blade (Oster Professional Products, McMinnville, TN) is ideal for fine, thin hair in patients with thin skin, such as small rodents. The teeth are closer together than those of the No. 40 blade, making it less likely that the patient’s skin will get caught between the teeth and be cut accidentally. Clear plastic drapes allow respiration to be monitored during the procedure. A plastic drape with a 3.5- × 5-in. adhesive center and an overall size of 40 × 54 in. is commercially available (Veterinary Transparent Surgical Drape, Veterinary Specialty Products, Inc.). This allows the surgeon to maintain a sterile field over the entire table and still monitor the patient. A smaller drape (24 × 24 in.) is also available for minor procedures but is not large enough to create a sterile field on a surgical table. These drapes may also hold heat better than cloth or paper drapes. Place the four quarter drapes around the patient rather than the proposed incision, then place the plastic drape over the patient to cover the entire table and create a sterile field.

Sutures, Needles, and Closure

In general, the sutures and needles in the average veterinary practice are too large for use in small rodents. It is best to use the smallest size suture and needle to accomplish the desired task. For these small patients, sizes 5-0 and smaller with short, fine, swaged-on needles are applicable. Cutting needles are mainly used for skin. Atraumatic taper-point needles are recommended for soft tissues. Monofilament rapidly absorbable suture is most appropriate for rodents (polyglecaprone, glycomer, polyglactin 910, and polyglycolic acid). Suture that is present for many months, such as polydioxanone and polyglyconate, is not usually needed in rodents.

The subcutis in small rodents is relatively thick and holds subcuticular sutures well. Rodents are fastidious groomers and tend to remove skin sutures. They also seem prone to self-mutilation of surgical wounds. Gentle tissue handling, strict attention to aseptic technique, and closing with no external sutures will help minimize the risks. Pinching the skin with forceps during suture placement causes bruising and irritation, predisposing to self-mutilation. In placing sutures, use forceps to provide counterpressure and guide the tissues without pinching. Chromic catgut should be avoided because of its reactive nature, which might stimulate self-mutilation and can delay healing. I prefer 5-0 to 7-0 rapidly absorbable suture in a subcuticular or intradermal closure to avoid the use of skin sutures. If necessary, tissue glue may also be used to facilitate skin apposition. Avoid using large amounts of tissue glue, because the patient will notice it and work to get it off, potentially resulting in self-trauma. The placement of intradermal sutures is time-consuming. If time is of the essence, small incisions can be apposed with only tissue adhesive or skin staples. Rodents tend to chew at skin sutures and can chew out wire sutures as well as nylon ones. Steel skin staples appear to be more resistant to being chewed out.